Audio Systems: Electronic Cross Over, electronic crossover, channel amplifier

Question
I am currently using one 4 channel amplifier with built in cross over to drive my sub woofer and 2 sets of bass mid and tweeter (one set each in rear and front). I was thinking of adding one 2 channel amplifier and one electronic crossover but was not advised to do so by quite a number of people. They claim my current amplifier is powerful eanough and it has already got a built in crossover and it will be a waste. however, they recommend me to install a pre amp to enable me to adjust the sound to my taste.

Please Advise. Thanks.  

Answer
If you want to add another amp for the bass, thats fine, that will allow your sub to play louder. I would get an map with an internal xover, that way you eliminate the need for buying that piece. A preamp is a good choice for tayloring your sound, but it wont make your system any louder
Tim